Multi-level vacuum pumping system for plasma containment device



MULTI-LEVEL VACUUM PUMPING SYSTEM FOR PLASMA CONTAINMENT DEVICE I of 4Sheet Fileq April 27, 1966 0 mm TA v mm vL 3w m V o I a' DE E mm N3 GR3N NNN ATTORNEYS July 1, 1969 CANN ET AL 3,453,469

MULTI-LEVEL VACUUM PUMPING SYSTEM FOR PLASMA CONTAINMENT DEVICE FiledApril 27, 1966 Sheet 2 of 4 GOR ONL. CANN 2 ROBERT HARDER A T TORNEYSJuly 1, 1969 1 CANN ET AL 3,453,469

MULTI-LEVEL VACUUM PUMPING SYSTEM FOR PLASMA CONTAINMENT DEVICE FiledApril 27, 1966 Sheet 5 of 4 A T TORNE July 1, 1969. CANN ET AL 3,453,469

MULTI-LEVEL VACUUM PUMPING SYSTEM FOR PLASMA CONTAINMENT DEVICE FiledApril 27, 1966 Sheet 4 of 4 I N VEN TORS' ORDON L. CANN G BY OiBOBEg L.EARDER $478 3 \M A T TORNEYS United States Patent Int. Cl. H01j 1/50 US.Cl. 313-7 1 Claim ABSTRACT OF THE DISCLOSURE Plasma containmentapparatus utilizing a multi-level vacuum pumping system which providesmore effective vacuum pumping by removing a portion of the material fromthe chamber at higher pressures than the prevailing chamber pressure isdisclosed. A plurality of flow restrictors and vacuum pumping means areemployed to produce the required difierential pressures.

This application is a continuation-in-part of our copending applicationbearing Ser. No. 458,837, filed May 20, 1965, and entitled Plasma ArcElectrodes.

This application relates to plasma containment devices and moreparticularly to improved vacuum pumping means and methods for usetherewith.

In our applications Ser. Nos. 457,414 and 457,746 plasma containmentdevices are disclosed comprising generally an evacuated chamber, aradial arc electrode assembly at one or both ends of the chamber withmeans to introduce ionizable material into the chamber through theseelectrode assemblies, and magnet coils to provide an axial magneticfield. A column of high temperature and pressure plasma extends down themagnetic field from the electrode assemblies, but proper operationrequires that the surrounding pressure in the vacuum chamber be keptvery low. Since ionizable material is continually being introduced intothe chamber at the electrodes a high capacity pumping system is requiredto remove all of this material from the chamber. It has now been foundpossible to remove some of this material at a pressure greater than theprevailing pressure in the vacuum chamber. This is highly desirablebecause less energy and less complex pumping equipment is required toremove a given mass of a material at a higher rather than a lowerpressure.

It is accordingly an object of the present invention to provide improvedvacuum pumping means and methods for use in plasma containment devices.It is a further object of the invention to provide vacuum pumping meansand methods whereby part of the gaseous contents of a vacuum chamber arepumped at a pressure higher than the prevailing chamber pressure.Further objects will become apparent with the description which follows.

FIGURE 1 is a schematic cross sectional view of a plasma containmentapparatus useful in connection with the present invention.

FIGURE 2 is a cross section of an electrode assembly utilizable incontainment devices of the type depicted in FIGURE 1.

FIGURE 3 is a partially schematic sectional view of one embodiment ofthe invention, and

FIGURE 4 is a sectional view of a further embodiment of the invention.

FIGURE 1 shows a form of plasma containment apparatus, more fullydescribed in our copending application 458,837 in which the presentinvention may be usefully employed. It includes a chamber 210 which isevacuated by pump 212 and which contains magnet coils ICC 214, 216, 218and 220 which are energized in the same direction by power supplies 222,224, 226, and 228. Water cooling may be provided for the magnet coils asshown by elements 230, 232 and 234. Located within coils 214 and 220 arearc electrode assemblies 236, 238, each of which includes at least acathode 240 and an anode 242 which are connected to a power supply 248as well as a gas supply channel 244, which is fed from a source 246 ofargon, hydrogen or other ionizable gas. The illustrated apparatus isparticularly adapted to form a confined, rotating column of hightemperature plasma extending from electrode assembly 236 to electrodeassembly 238 and having an internal radial electric field. A relatedform of plasma containment apparatus, more fully described in thecopending application 457,414 of the joint applicant Gordon L. Cann,employed only a single electrode assembly 236 or 238.

FIGURE 2 is a cross sectional view of an electrode assembly useful aselement 236 or 238 in FIGURE 1. Cathode 10 is a tapered piece oftungsten and, like all other electrodes in the figure, will normally beaxially symmetric. It is mounted in a metal heat sink 14, which, inturn, is mounted at the end of a cathode support and cooling waterconduit 21 which is sealed into phenolic support block 18. A cathodecooling water inlet, not shown, communicates with chamber 24 and in turnwith an unshown outlet to provide water cooling for cathode 10. A boronnitride or beryllia insulator 26 surrounds cathode 10 and leaves the tipportion exposed. A concentric cathode bufier electrode 28, surroundscathode 10 and is supported with respect to the cathode and insulatedtherefrom by insulator 26. As shown, the cathode buffer 28 is taperedinternally and defines a chamber 40 surrunding the tip portion of thecathode which is substantially enclosed except for an aperture 30 in thecathode buffer, which is aligned with cathode 10 and positioned slightlyin front of the cathode tip. illustratively, the diameter of thisaperture may be 0.1 inch.

Cathode 10 and its heat sink 14 are bored to receive a tubular pressuretap 32 located within the cathode water conduit 21. Cathode 10 alsocontains one or more small channels or passage 34 which connect thepressure tap to the exterior surface of the cathode forward of thecathode insulator 26. The cathode insulator 26 is also provided with agas passage, or preferably a plurality of circumferentially disposedpassages 36 which communicate with the front face of the cathodeinsulator and are connected to a feed tap 38 in support block 18. Eitherpassages 34 or passages 36 may be used to introduce a fluid to the spaceadjacent to the cathode, but it is generally preferable to introduce afluid through passages 36, and to use passages 34 for measuring thepressure adjacent to cathode 10.

Tungsten cathode buffer electrode 28 is attached to and is in thermalcontact with a hollow heat sink and cooling assembly 50 which isconnected to an electrically conducted water inlet tube 54. Thecorresponding water outlet is not shown.

Cathode buffer electrode 28 is surrounded by a series of alternatingconcentric electrodes and insulators 122. The outermost insulators 122serves to insulate and support a tungsten anode buffer electrode 60,which is concentrically located about the cathode and cathode buffer. Awater cooled copper anode assembly 70 is mounted on the outside ofsupport block 18 and is electrically insulated from anode buffer 60 andheat sink 50 by insulators 72 and 74. Anode 70 has at its forward end acylindrical inner surface 76 which illustratively may be 2 inches indiameter and is separated by a small annular space from a cylindricalouter surface of anode buffer 60. Illustratively, the forward surfacesof the cathode buffer 28, electrodes 20, insulators 122, anode bufferelectrode 60, and anode 70, may lie on a common plane as shown. There isa radial passage 84 in anode 70 which opens into the annular space 78and communicates with an anode pressure tap 86. A magnet coil 90 isshown which is insulated from the anode 70 by an insulator 88 whichsurrounds the anode and also covers the front face thereof. Insulator 88also prevents arc attachment to the face of the anode, which would causevery rapid erosion. A conventional power supply may be used to operatemagnet 90. A water cooling assembly 94 is positioned so as to cool theforward portion of the magnet and also the face of the anode, each ofwhich is likely to be exposed to high temperatures in the operation ofthe device. A suitable DC power supply 100 and switch are connectedbetween cathode cooling conduit 21 and anode assembly 70.

A series of passages 124, extend outward from cathode chamber 40,passing through electrodes 120 and insulator 122. These passages do notlie in planes passing through the axis of the device, but are canted togive a tangential velocity to gas issuing therefrom in a directionconsistent with the magnetic field. A tube 82 connects a vacuum pump 126with annular space 78. Vacuum pump 126 has a beneficial effect when usedin conjunction with the containment device of FIGURE 1. It is desirableto have a relatively high mass flow of gas through cathode chamber 40and passages 124 to assist in cooling the electrodes. However, this gasmust be somehow removed from the chamber containing the apparatus inorder to permit continuous operation. Much less energy is expended inremoving this gas at the relatively high pressure encountered at space78 than at the relatively lower pressure encountered in chamber 210.

FIGURE 3 shows one half of an embodiment of the invention correspondingto an actual experiment. However, it will be understood that no attempthas been made to show all structural details such as cooling, passages,fasteners, shields, welds, and the like. The omitted portion of theapparatus may be a mirror image of the portion shown. The vacuum chamberconsists of a central tube 150, approximately 30 inches along with aninside diameter of 3.9 inches connected through insulator 152 to vacuumend chamber 154. This configuration permits magnets 156 to be placedoutside the vacuum rather than inside as shown in FIGURE 1. Central tube150 is preferably made of water-cooled copper to permit dissipation oflarge amounts of heat and is electrically floating with respect to endchamber 154. Except for the need to dissipate heat, tube 150 could bemade of ceramic or other insulating material. Electrode assembly 36 isgenerally similar to the one described in connection with FIGURES 1 and2 except that it includes a water-cooled magnet coil 90 which performs afunction similar to the end coils 214 and 220 in FIGURE 1 in extendingthe magnetic field through the electrode assembly, and includes anelectrically floating anode shield 160 placed immediately in front ofthe anode 70. The latter element is more fully described in anapplication by the present inventors filed approximately simultaneouslywith the instant one and entitled Plasma Arc Electrodes With Anode HeatShield, Ser. No. 545,701, filed Apr. 27, 1966.

Each end chamber 154 and thus central tube 150, is evacuated by a pump12. A single pump may be connected and paralleled to both end chambers.A water cooled vacuum separator 100, surrounds electrode assembly 236and includes an aperture 102 positioned a short distance in front of andin alignment with anode shield aperture 101. Vacuum separator 100 issupported and electrically insulated from chamber 154 by insulator 153.Vacuum separator 100 defines an inner chamber 106 which does notcommunicate with end chamber 154 except through aperture 102 which isslightly larger than the anode diameter. Inner chamber 106 is connectedto a pump 110 which is separated and distinct from pump .4 12. This pumpcan be connected to evacuate both inner chambers 106.

When the illustrated apparatus is operated in the manner described inconnection with FIGURE 1, a plasma column is formed at electrodeassembly 236 and extends into central tube 150. It is believed thatthere is a longitudinal flow of plasma along the outside of the plasmacolumn and in the direction of the electrode assembly. This flow ispassed by aperture 102 but intercepted by the anode heat shield 160 orthe anode itself. Regardless of theory, it has been found that thepressure in inner chamber 106 will greatly exceed the pressure outsidethe inner chamber and a large portion of the gas flow which must beevacuated from the device can be withdrawn by pump 110 at a much highersuction pressure than exists at the inlet to pump 12. As previouslyexplained, it is much more efficient to remove gas at a higher ratherthan a lower pressure. In addition, vacuum separator 100 andparticularly the area surrounding aperture 102 serves to absorb someheat which would otherwise have to absorbed by other parts of theapparatus already burdened by a high heat load. The portion of vacuumseparator 100 surrounding aperture 102, can be constructed in the samemanner described for a similar element in our simultaneously filedapplication relating to improvements in Plasma Arc Electrodes With AnodeHeat Shield, Ser. No. 545,701, filed Apr. 27, 1966.

In one experiment, utilizing the embodiment of FIG- URE 3, .0237 gramper second of hydrogen was introduced at each electrode assembly, themagnet coil and are assemblies being de-energized. Under theseconditions the pressure inside inner chamber 106 was 100 microns and thepressure outside was microns, the difference being presumably caused bya higher pumping capacity of pump 110 as compared to pump 12. When aplasma column was formed by energizing magnets 156 and 90, to create a20,000 gauss field in tube and an 8,000 gauss field at the electrodeassembly, and 200 amperes of current at 68 volts was forced between eachcathode and anode, the pressure in the inner chamber 106 rose to 550microns while the pressure outside was 200 microns. This clearlydemonstrates that the pressure increase inside inner chamber 106 iscaused by the dynamics of the plasma discharge and not simply due todifferences in pumping capacities of the two pumps nor to the fact thatthe feed gas is introduced to the system inside chamber 106. In adifferent experiment, the hydrogen flow rate was increased slightly to.028 gram per second, the magnetic field inside tube 150 was increasedto 26,000 gauss and the magnetic field at the electrode assembly wasincreased to approximately 9,500 gauss. Under these conditions, thepressure within chamber 106 was 1900 microns while the pressure outsideremained at 220 microns. This represents a pressure ratio of 8.6 betweenthe pressure existing generally in tube 150 and end chamber 154 and thatwithin the inner chamber 106.

FIGURE 4 shows in a highly schematic fashion a still further embodimentof the invention. The apparatus is generally similar to that of FIGURE 3except that pumping ports or plenums are defined in tube 150 by circularrestrictors provided between some of the magnet coils. The circularrestrictors, preferably water-cooled metal, are seen to be adjacent eachport on the side of the port nearest the outer end of the device. Theouter restrictors 200 have the smallest internal diameter, intermediaterestrictors 202 have a larger diameter, and inner restrictors 204 havethe largest diameter. Restrictors 200 are functionally similar to vacuumseparators 100 in FIGURE 3. Each plenum or port is connected with itsown vacuum pumping system, except the corresponding symmetricallypositioned ports may be connected to a common pump as shown. Thusplenums 205 are connected to a common pump 110 and plenums 206 areconnected to common pump 208. In a similar manner the two outletsserving plenums 210 are connected to a common pump 212 and the twooutlets serving plenums 214 are connected to a common pump 216. Eachrestrictor intercepts a portiton of the longitudinal flow of plasmaalong the outside of the plasma column contained in tube 150 duringoperation of the device. The plasma recombines and is neutralized at thesurface of the restrictors and is removed through the associated pumpingsystem. In this way the total pumping effort required to maintain thevacuum chamber evacuated is distributed over a large number of pumps.The distribution of pumping load among the various pumps can becontrolled through variation of the diameters of the variousrestrictors. In FIGURE 4 pump 216 must be adapted to pump against a lowsuction port pressure. However, the pump inlet pressure will beprogressively higher at pump 212 and 208 and will be highest at pump 110which will also handle the largest mass flow. In this way the materialnecessarily introduced into the plasma through feed passages 36 isremoved in the most efiicient possible way. Furthermore, much of thethermal energy in the plasma which would otherwise be dissipated at ornear the electrode assemblies is removed by the various restrictors. Byspreading the thermal load over more elements, the load on each one isreduced and operation at higher power levels becomes possible.

While the present invention has been particularly described in terms ofspecific embodiments thereof, it will be understood that in view of thepresent disclosure numerous modifications thereof and deviationstherefrom may now be readily devised by those skilled in the art withoutyet departing from the present teaching. Thus, for example, symmetryabout a linear axis is preferred, but departures from this symmetry canbe tolerated and the axis itself can be curved since the discharge willfollow the magnetic field whether straight or not. Similarly themagnetic coils can be other than solenoidal or cylindrical, as by beingwound on conical forms, and permanent magnets may be used within thelimits of their technology. Other variations in size, configuration,operating conditions and the like are encompassed by the invention andthe invention is accordingly to be broadly construed and limited only bythe spirit and scope of the claim now appended hereto.

What is claimed is:

1. Plasma containment apparatus comprising:

(a) a vacuum chamber;

(b) first vacuum pumping means to evacuate said vacuum chamber;

(c) magnetic means to form a longitudinally continuous magnetic fieldalong a line within said chamber; (d) at least one plasma arc generatordisposed within said magnetic field on said line and substantiallysymmetrical thereabout, said generator comprising a central cathodeelectrode and an anode electrode encircling said cathode, said generatorincluding at least one passage terminating between said cathode andanode, gas supply means to introduce a plasma forming gas through saidpassage and power supply means to maintain an arc discharge between saidanode and said cathode;

(e) multiple, annular, axially positioned flow restrictor meanspositioned on said line intermediate said first pumping means and saidgenerator, said flow restrictor means sealed to said chamber butelectrically insulated therefrom, said annular restrictor means havinginternal apertures successively larger in accord with the increasingdistance of said restrictor means from the nearest plasma arc generator,said restrictor means being adapted to impede the flow of gaseous matterto said first pumping means; and,

(f) second pumping means comprising individual pumping means associatedwith each of said restrictors, each of said pumping means beingpositioned to evacuate volumes of said chamber adjacent the side of saidrestrictor associated therewith in closest proximity to the nearest arcgenerator.

References Cited UNITED STATES PATENTS 4/1959 Beam et al 313-7 10/1961Dandl 313231 X US. Cl. X.R.

1 UNI'IEI) S'IA'IES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,453,460 Dated July 1, 1969 Inventor(s) (iordon I. Cann It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

In the drawings, Sheets 1-4:

(a) At the top of each sheet, cancel "G. L. CANN ET AL" and substitutetherefor -G. L. CANN-;

(b) At the lower righthand corner of each sheet;

cancel "INVENTORS" and substitute therefor -INV ENTOR; and cancel"ROBERT L. HARDER".

Column 1, lines 4 and 5, cancel "and Robert L. Harder, Altadena, Calif.assignors" and substitute therefor -assignor.

SIGNED AND SEALED MAY 261970 (SEAL) Attest:

WILLIAM E. suauYwR. EdwardM. Fletcher, 11'. Commissioner of PatentsAttesting Officer

